The present invention relates to an engine piston, and more particularly relates to an engine piston of which a part at least is composite reinforced by inorganic fibers, preferably alumina/silica fibers.
In an internal combustion engine the piston or pistons thereof are generally subject to very severe operational conditions, and as a result the provision of a strong constitution therefor is very important. In particular, the two important design characteristics that are very important for an engine piston are resistance to wear and resistance to burning or seizure. Now, the part of a piston that is most subject to wear is the upper and lower wall surfaces of the top ring receiving groove thereof, because these surfaces slide under high pressure and temperature conditions and inevitably often under rather poor lubrication conditions against the top piston ring; while the part of a piston that is most subject to burning and seizure is the top land portion thereof, i.e. the part of the side wall surface of the piston between the top or crown thereof and said first ring groove, because this part of the piston is not only almost directly exposed to the flame within the cylinder and the exhaust gases while being rather poorly cooled by conduction to the cylinder side wall, but also slides along or close to said cylinder side wall at high speed, again inevitably often under rather poor lubrication conditions. These conditions of severe wear and severe burning and/or seizure are aggravated in the case of a diesel internal combustion engine, because of the higher compression ratio thereof, which entails generally a more severe cylinder environment.
Accordingly, in the prior art it has been widely practiced, in the case of diesel engines, for the part of the piston from which the top piston ring groove is formed to include a reinforcing insert formed of special cast iron such as Niresist cast iron, and since the wearing characteristics of such a form of cast iron are better than the wearing characteristics of the sort of metal from which such a piston is mainly constructed, such as a light alloy such as for example aluminum alloy, thereby the piston is made to be more resistant to wearing on its portion (the side wall surfaces of the top ring groove) which is most subject to such wear, while still being mainly made of a material such as the light alloy.
However, such pistons with cast in anti wear rings made of special cast iron such as Niresist cast iron have certain defects, as follows. First, special cast iron such as Niresist cast iron has a much higher specific gravity than does a light alloy such as for example aluminum alloy, and therefore a piston including a cast in anti wear ring of special cast iron such as Niresist cast iron is necessarily rather heavier than a piston made of light alloy such as aluminum alloy only. Although this does not present any particular problems in the case of a diesel engine for low or medium speed use, on the other hand in the case of a diesel engine for high speed use, or in the case of a gasoline engine, it presents problems with regard to the attainment of higher performance. Second, the thermal conductivity of special cast iron such as Niresist cast iron is much lower than that of a light alloy such as for example aluminum alloy (in fact, is between one fifth and one tenth thereof), and therefore a piston including a cast in anti wear ring of special cast iron such as Niresist cast iron has much poorer heat dissipation characteristics, since a significant part of the cooling of a piston is accomplished by conduction of heat from the crown of the piston via the piston rings to the side wall of the combustion cylinder, and accordingly said cooling takes place rather poorly via such an anti wear ring of special cast iron such as Niresist cast iron, if one in fact is provided. This means that in the case of a high performance engine, in which the thermal load on the piston is high, a danger exists of a buildup of heat in the crown portion of the piston, which can lead to melting thereof, or to seizing of the piston in the cylinder, or to burning damage and the like. Third, special cast iron such as Niresist cast iron is expensive. Fourth, special cast iron such as Niresist cast iron is difficult to work. Finally, fifth, since special cast iron such as Niresist cast iron has a poor affinity for light alloy such as for example aluminum alloy, this necessitates special processing for such a cast in anti wear ring formed of such special cast iron, such as so called Alfin processing. As a result of all this, a piston including such a cast in anti wear ring made of special cast iron is expensive and is not fully satisfactory in use.